Load protective device including positive temperature coefficient resistance



March 15, 1966 N H 3,241,026

LOAD PROTECTIVE DEVICE INCLUDING POSITIVE TEMPERATURE COEFFICIENTRESISTANCE Filed Dec. 5, 1962 FIG. 2

INVENTOR.

EKKEHARD ANDRICH BY M16.

United States Patent 1 3,241,026 LOAD PROTECTIVE DEVICE INCLUDING POSITIVE TEMPERATURE COEFFICIENT RESIST- ANCE Ekkehard Andrich, Aachen,Germany, assignor to North American Philips Company, Inc., New York,N.Y., a corporation of Delaware Filed Dec. 5, 1962, Ser. No. 242,388Claims priority, application Germany, Dec. 8, 1961, N 20,931; Nov. 8,1962, N 22,327 13 Claims. (Cl. 318-442) This invention relates toprotective devices, and more particularly to a device by means of whicha load may be automatically protected against overload conditions. Bymeans of the present invention a load device such as the miniature motorin a shaving apparatus may be safely and reliably connected to two ormore different supply voltages. A common arrangement used in the pastincludes a current limiting resistor which is usually arranged in aseries circuit between the supply source and the load. In thisarrangement the resistor can be short circuited by means of a mechanicalswitch when the device is connected to the lower supply voltage. Theresistor and the switch are usually provided in the plug of theapparatus.

These devices suffer from the limitation that if the user of theapparatus, when connecting it to the higher supply voltage, forgets toopen the shortcircuit of the current limiting resistor, the resultingcurrent surge may overload the apparatus as a result of which it couldbecome defective.

According to the present invention, the function of the switch isautomatically performed by a current dependent resistor. The inventionis characterized by the employment of the parallel combination of afixed ohmic resistor whose resistance is substantially independent oftemperature and a resistor having a high positive temperaturecoefficient (P.T.C.) of resistance. The expressions fixed resistor andresistor having a high positive temperature coeflicient hold at least inthe voltage range of the said supply voltages. This parallel combinationis arranged in a series circuit between the source of supply voltage andthe load. In particular, the current dependent resistor has so high apositive temperature coefiicient of resistance that, when connected tothe highest supply voltage available, it operates in the negativeresistance region of its current-voltage characteristic curve. In fact,it is preferable that when the current dependent resistor is connectedto the higher {supply voltage, it operates at a considerably lowercurrent than when it is connected to the lower supply voltage. The widevariation in resistance of the current-dependent resistor when connectedto the higher or lower supply voltage in effect acts as an automaticelectronic switch.

It is also known to connect an iron resistor having a rather largepositive temperature coefiicient in series with the load, therebylimiting the load current to a predetermined safe value. Such a resistorconsequently operates as a current limiter and not as an electronicswitch. Undesirable currents in this resistor and temperature variationswill influence the current through the load to a considerable extent,which in turn adversely affects the reliability of the device. In thedevice in accordance with the invention, a considerable portion of theload current flows through the fixed resistor connected in parallel withthe FTC. resistor. Therefore, the reliability of the device has beenconsiderably increased. A further difference with respect to the ironresistors previously used consists in that the P.T.C. resistors used inthe practice of this invention exhibit a high positive temperaturecoefficient only above a given temperature (for example 100 C.) and froma given power, respectively.

Patented Mar. 15, 1966 Below this temperature, that is to say as low asroom temperature, this resistor may be considered to be an ohmicresistor which is substantially independent of temperature. However,iron resistors usually exhibit a positive temperature coefficient evenat room temperatures. A method of fabricating a P.T.C. resistorparticularly adapted for use with this invention is described and shownin the British specification 714,965.

It is therefore an object of this invention to provide an improvedprotective arrangement which permits a motor or other device to besafely and reliably connected to two or more different sources of supplyvoltage.

It is also an object of this invention to provide an overload protectionarrangement which is of extremely simple construction and is reliable inoperation.

In order that the invention may be readily carried into effect, it willnow be described with reference to the accompanying drawing in which:

FIG. 1 shows a device in accordance with the invention;

FIG. 2 illustrates several waveforms helpful in understanding theinvention including the current-voltage characteristic of a resistorhaving a positive temperature coefficient and the characteristic of theparallel combination of such a resistor and a fixed resistor which maybe used in the device shown in FIG. 1.

FIG. 3 shows another embodiment of the invention.

FIG. 4 illustrates still another embodiment of the invention.

The device shown in FIG. 1 comprises a load 1, for example, theminiature motor of an electric shaving apparatus, which may arbitrarilybe connected to a higher alternating supply voltage, for example, of 220volts, and to a lower alternating supply voltage, for example, volts,respectively. In the interests of simplicity, the motor field windingshave not been illustrated. To this end, according to the invention, theparallel combination of a fixed resistor 3 and a resistor 4 having ahigh positive temperature coefiicient is provided in series between theload 1 and the supply voltage 2. The resistor 4 has a current-voltagecharacteristic I-V as indicated in FIG. 2 by curve 4. Thecurrent-voltage characteristic of the parallel combination of resistors3 and 4 as indicated in FIG. 2 by the curve labelled 3+4.

The circuit arrangement of FIG. 1 is proportioned so that at the lowersupply voltage the parallel combination of resistors 3 and 4 is operatedat the operating point A, whereas .at the higher supply voltage it isoperated at the operating point B, FIG. 2, 'The straight lines 1 and 1'in FIG. 2 indicate the characteristic of the load 1 when connected tosupply voltages of 110 volts and 220 volts, respectively.

In a particular embodiment of the invention, the load 1 has a resistanceof, for example, 2,000 ohms. The value of the resistor 3 is, forexample, 3,300 ohms. At the lower supply voltage of 110 volts, theresistor 4 has a resistance value of 200 ohms. When the device isconnected to the higher supply voltage (220 v.) the resistance ofresistor 4 has increased to a value of about 20,000 ohms and a loadcurrent flows in the circuit which corresponds to the operating point B.As can be seen from the operating point B of the resistor 4 alone, thiscurrent has only increased to a slightly higher value than that of theresistor 3, and the small residual current actually flowing throughresistor 4 has only a slight influence on the total current through theload 1.

The circuit arrangement described has the additional advantage that inthe event the rotor of the motor 1 is braked too strongly, resulting ina sharp reduction in the counter E.M.F. produced by the motor, thecurrent flowing through the motor is prevented from increasing to adangerously high value. In other words, the variation of the resistancevalue of the P.T.C. resistor 4 also acts as an automatic electricoverload protection in the event of a sudden braking of the motor.

In order to obtain further overload protection for the device againstthe possible connection to an even higher supply voltage of, forexample, 265 volts, another P.T.C. resistor 5, as shown in FIG. 3, maybe added in series with the elements previously described. The latterresistor may also be provided instead of the resistor 4, if itsresistance value and its maximum power dissipation are such that itfulfils the condition for operating point B of FIG. 2. In other words,replacing the series combination of resistors 4 and 5 by a singleresistor 5 requires that resistor 5 must still remain ohmic at thisoperating point, and that it only reaches its P.T.C. range at a higherpower level.

It is of course evident that the arrangements shown in FIGS. 1 and 3 arenot limited to use with the motor load described, but may be used inconnection with any arbitrary load requiring a similar automatic type ofprotection. Whereas in shaving apparatus the protective device isusually provided in the plug, a device in accordance with the inventionmay readily be arranged in the housing of the load itself and thermallyisolated therefrom. Consequently, it is possible to use a conventionalplug which results in the total arrangement becoming less expensive.

The following is a description of an improvement of the devicedescribed.

It has appeared that the load resistance, more particularly the internalresistance of the motor, may show considerable resistance variationswhich endanger the reliability of the device. Consequently, if thedevice is proportioned so that for one value of the load resistance itrenders a reliable switching of the operating point possible whenswitching from one supply voltage to the other, it may happen that whenconnecting a load with a comparatively low internal resistance to thelower supply voltage, the maximum in the I-V characteristic of theP.T.C. resistor is exceeded and a wrong operating point is obtained. If,in order to prevent this danger, the P.T.C. resistor is proportioned so,that the maximum in its I-V characteristic lies comparatively high,again the danger exists that when connecting a load with a comparativelyhigh internal resistance to the higher supply voltage, this maximum isnot exceeded and too large a voltage is set up at the load.

According to a further feature of the invention, these difiiculties maybe avoided by connecting parallel to the load, a voltage-dependentresistor having a strongly increasing current conductivity in thevoltage range between the values of the two supply voltages. For thispurpose, parallel to the load 1 a voltage-dependent resistor 6 (V.D.R.),viz. FIG. 4, is connected, the conductivity of which strongly increasesin the range between the values of the lower and the higher supplyvoltage. In addition, the P.T.C. resistor 4 is proportioned so that themaximum in the IV characteristic lies considerably higher than the pointof intersection A in FIG. 2. Its characteristic is represented by thecurve 4a and the total characteristic of resistors 3 and 4 by the curve3+4a in FIG. 2. A load having a considerably smaller internal resistancethan corresponds to the straight line 1 in FIG. 2 will never be capableof exceeding the maximum in the curve 3+4a when connecting the device tothe lower supply voltage and consequently it will result in a correctoperating point.

However, if the load resistance is larger than corresponds to thestraight line 1, without the resistor 6 the danger exists that whenconnecting the device to the higher supply voltage the correspondingstraight line ll intersects the curve 3+4a in the proximity of thecurrent maximum, so that no switching to the operating point B wouldoccur. In that case the voltage at the resistors 3-4 would be small andthat at the load would consequently be inadmissibly high.

This latter voltage is so large that when connecting the device, theresistor 6 is immediately operated at an operating point where itscurrent permeability has increased strongly. Its correspondingresistance then may be, for example, only 1000 ohms and the I-Vcharacteristic of the parallel combination 1, 6 will then correspond,for example, the curve 1+6 in FIG. 2. In that case the current flowingin the device rises above the maximum in the I-V characteristics 3+4-aand this current will rapidly heat the resistor 4 until an operatingpoint is reached which corresponds approximately to the point B. As aresult of this, however, the voltage drop at the resistors 34 increasesso strongly that the voltage at the load 1 and at the resistor 6respectively is again reduced to the required lower value. However, atthis voltage the resistor 6 again has a high value, for example, a fewtimes 10,000 ohms, so that it causes only a slight additional currentload on the supply source.

In a practical example, with the above proportioning of the circuitelements 1, 3 and 4, the resistor 6 was chosen such that at v. it had avalue of 27,500 ohms. However, at 220 v. it had a value of 1,000 ohms.

Although the invention has been described with reference to threespecific embodiments, its principles have been set forth in sufiicientdetail to enable those skilled in the art to design and construct otheruseful apparatus embodying these principles.

What is claimed is:

1. Protective apparatus for an electrical load device adapted forconnection to a source of electrical energy having first and secondvoltage levels, said second voltage level being higher than said firstvoltage level, comprising a pair of parallel connected resistanceelements serially connected with said energy source and said loaddevice, one of said resistance elements having a positive temperaturecoetficient of resistance above a predetermined voltage level, saidpositive temperature coeflicient resistance element undergoing avariation in resistance between said first and second voltage levelswhich produces a lower value of current flow therein at said secondvoltage level than occurs therein at said first voltage level and theother resistance element having a small temperature coefiicient ofresistance relative to the temperature coefficient of said oneresistance element.

2. Protective apparatus for an electrical load device adapted forconnection to a source of electrical energy having first and secondvoltage levels, said second voltage level being higher than said firstvoltage level, comprising a pair of parallel connected resistanceelements serially connected between said energy source and said loaddevice, one of said resistance elements having a high positivetemperature coeflicient of resistance above a predetermined voltagelevel and a low temperature coefficient below said predetermined voltagelevel, said predetermined voltage level occurring at a voltage level ofsaid energy source which is intermediate said energy source first andsecond voltage levels, and the other of said resistance elements havinga small temperature coefficient of resistance relative to thetemperature coetficient of said one resistance element.

3. Protective apparatus for an electrical load device adapted forconnection to a source of electrical energy having first and secondvoltage levels, said second voltage level being higher than said firstvoltage level, comprising a pair of parallel connected resistanceelements serially connected with said energy source and said loaddevice, one of said resistance elements having a positive temperaturecoeflicient of resistance above a predetermined voltage level which isintermediate said energy source first and second voltage levels and theother resistance element having a temperature coefficient of resistancewhich is small relative to the temperature coeffieient of id one re i tnce elemen and a voltage dependent resistor connected in parallel tosaid load device, said voltage dependent resistor exhibiting a sharplyincreasing conductivity above a predetermined voltage level intermediatesaid energy source first and second voltage levels.

4. Protective apparatus for an electrical load device adapted forconnection to a source of electrical energy having first and secondvoltage levels comprising a pair of parallel connected resistanceelements serially connected between said energy source and said loaddevice, one of said resistance elements having a region exhibiting asubstantially linear current versus voltage characteristic below apredetermined voltage level and having a negative resistance regionabove said predetermined voltage level, said predetermined voltage levelbeing intermediate in value said energy source first and second voltagelevels, and the other resistance element having a value of resistancewhich is substantially independent of temperature at said first andsecond voltage levels.

5. Protective apparatus for an electrical load device adapted forconnection to a source of electrical energy having first and secondvoltage levels, said second voltage level being higher than said firstvoltage level, comprising first and second .parallel connectedresistance elements connected in series circuit with said energy sourceand said load device, said first resistance element having asubstantially constant temperature coefficient of resistance at saidfirst voltage level and a high positive temperature coefficient ofresistance at said second voltage level, and said second resistanceelement having a resistance value substantially independent oftemperature at said first and second voltage levels, said firstresistance element undergoing a variation in resistance such that asubstantially lower current fiows therein at said second voltage levelthan flows therein at said first voltage level.

6. Protective apparatus for an electrical load device adapted forconnection to a source of electrical energy having first and secondvoltage levels, said second voltage level being higher than said firstvoltage level, comprising first and second parallel connected resistanceelements, a third resistance element connected in series circuitarrangement with said parallel connected resistance elements, the lattercircuit being serially connected with said energy source and said loaddevice, said first resistance element having a positive temperaturecoefiicient of resistance above a first predetermined voltage level,said second resistance element having a temperature coefiicient ofresistance which is small relative to the temperature co eificient ofsaid first resistance element, and said third resistance element havinga positive temperature coefficient of resistance above a secondpredetermined voltage level which is higher than said firstpredetermined voltage level.

7. Apparatus for protecting an electrical load device adapted forconnection to a source of electrical energy from an overload conditioncomprising first and second parallel connected resistance elementsserially connected with said energy source and said load device, saidfirst resistance element having a high positive temperature coefiicientof resistance above a predetermined applied voltage level such that itexhibits a negative resistance characteristic above said voltage level,said second resistance element having a relatively small temperaturecoetficient of resistance, said first resistance element undergoing asharply increasingly value of resistance above said voltage levelthereby to limit the load current to said load device to a safe value inthe event said predetermined voltage level is exceeded.

8. Apparatus as described in claim 7 wherein said load device comprisesthe armature of an electric motor and said overload condition resultsfrom a stalled condition of the motor.

9. Protective apparatus for an electrical load device adapted forconnection to a source of electrical energy having first and secondvoltage levels, said second voltage level being higher than said firstvoltage level, comprising a pair of parallel connected resistanceelements serially connected with said energy source and said loaddevice, one of said resistance elements having a positive temperaturecoeflicient of resistance above a predetermined voltage level and theother resistance element having .a relatively small temperaturecoefiicient of resistance at said first and second voltage levels, and aresistance element connected in parallel with said load device andhaving electrical resistance variations as determined by variations inthe voltage applied thereto.

10. Apparatus as described in claim 9 wherein said voltage dependentresistance element has a high electrical resistance value upon applyinga relatively low voltage thereto and a low electrical resistance valueupon apply-ing a high voltage thereto.

11. Protective apparatus for an electrical load device adapted forconnection to a source of electrical energy having first and secondvoltage levels, said second voltage level being higher than said firstvoltage level, comprising first and second parallel connected resistorsserially connected with said energy source and said load device, saidfirst resistor exhibiting a positive resistance characteristic at saidfirst voltage level and a negative resistance characteristic at saidsecond voltage level, and said second resistor has a relatively smalltemperature coefi'icient of resistance.

12. Apparatus as described in claim 11 wherein said second resistor hasa substantially fixed value of resistance which is substantiallyindependent of temperature at said first and second voltage levels ofsaid energy source.

13. Apparatus as described in claim 11 wherein said load devicecomprises the armature winding of an electric motor.

References Cited by the Examiner UNITED STATES PATENTS 1,094,733 4/1914Lyle 317-41.1 1,225,388 5/ 1917 Woodbridge 317-41.1 2,086,910 7/ 1937Hansell 323-66 2,100,854 11/1937 Kaufmann 31741.1 2,332,073 10/ 1943Grierson 323-66 2,366,992 1/ 1945 Willing et al. 317-41.1 2,476,330 7/1949 Sitzer 317-41 2,724,761 11/ 1955 Cisne 323-94 2,747,158 5/1956LeBel 338-20 3,025,455 3/ 1962 Jonsson 323-94 CRIS L, RADER, PrimaryExaminer.

1. PROTECTIVE APPARATUS FOR AN ELECTRICAL LOAD DEVICE ADAPTED FOR CONNECTION TO A SOURCE OF ELECTRICAL ENERGY HAVING FIRST AND SECOND VOLTAGE LEVELS, SAID SECOND VOLTAGE LEVEL BEING HIGHER THAN SAID FIRST VOLTAGE LEVEL, COMPRISING A PAIR OF PARALLEL CONNECTED RESISTANCE ELEMENTS SERIALLY CONNECTED WITH SAID ENERGY SOURCE AND SAID LOAD DEVICE, ONE OF SAID RESISTANCE ELEMENTS HAVING A POSITIVE TEMPERATURE OF COEFFICIENT OF SAID RESISTANCE ABOVE A PREDETERMINED VOLTAGE LEVEL, SAID POSITIVE TEMPERATURE COEFFICIENT RESISTANCE ELEMENT UNDERGOING A VARIATION IN RESISTANCE BETWEEN SAID FIRST AND SECOND VOLTAGE LEVELS WHICH PRODUCES A LOWER VALUE OF CURRENT FLOW THEREIN AT SAID SECOND VOLTAGE LEVEL THAN OCCURS THEREIN AT SAID FIRST VOLTAGE LEVEL AND THE OTHER RESISTANCE ELEMENT HAVING A SMALL TEMPERATURE COEFFICIENT OF RESISTANCE RELATIVE TO THE TEMPERATURE COEFFICIENT OF SAID ONE RESISTANCE ELEMENT. 